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Ask most people what holds a skyscraper up and they think about its weight. Fazlur Rahman Khan saw that for very tall buildings the harder problem is the wind trying to tip them over. His answer was to make the outside of the building behave like one stiff hollow tube. This post covers who he was, what a tube structure is, a worked example with torque, the mistakes students make, and what you can take from his way of working.
Who was Fazlur Rahman Khan?
The University of Illinois says he was born in 1929 in East Bengal in British India, now Bangladesh, to a father who taught mathematics. He came to Illinois on a Fulbright scholarship and in three years earned two master's degrees, in theoretical and applied mechanics and in structural engineering, and a PhD in structural engineering. The university's alumni award citation dates the PhD to 1955 and says he joined the Chicago firm Skidmore, Owings & Merrill (SOM) that year, later becoming a partner and head of its structural division.
Princeton University's exhibition Fazlur Khan: Structural Artist of Urban Building Forms gives his dates as 1929 to 1982 and says that from 1965 to 1982 he significantly advanced the design of tall buildings. He worked closely with the architect Bruce Graham, who shared Khan's principle that "good architecture must also be good engineering and particularly good structure." He also taught students at the Illinois Institute of Technology.
Why is wind the real problem for tall buildings?
Princeton's page on the John Hancock Center explains that in tall buildings the controlling load is usually the sideways push of wind. Khan showed that as a building gets taller, the extra material needed to resist that push grows quickly. He called this extra material the building's "structural height premium" and argued that each range of heights needed its own kind of structure. For buildings above about 60 storeys, he proposed that all the outer wall elements should act together like the sides of a tube.
What are framed, trussed and bundled tubes?
Framed tube. Closely spaced outer columns tied by stiff beams, first used, Princeton says, in Chicago's DeWitt-Chestnut apartments. SOM's project page lists it as a 42-storey, 394-foot building completed in 1965. Princeton notes it behaved partly like a tube and partly like an ordinary frame.
Trussed tube. Giant diagonal braces on the outer walls make the tube act more fully as one unit. The 100-storey, 1,127-foot John Hancock Center, led by Graham and Khan, shows the diagonals on its face.
Bundled tube. Princeton's Sears Tower page describes nine square tubes, each 75 feet across, in a three-by-three grid, with columns 15 feet apart and shared walls between neighbours. Some tubes stop at lower levels. Princeton says the system saved Sears about $10 million compared with existing structural systems. The Illinois civil engineering department notes that the tower opened in 1973 and was the world's tallest building until 1998.
Worked example: why put the columns on the outside?
This is a simplified model with sample numbers. A building is 30 m wide and 200 m tall, and wind pushes on one face with a uniform pressure of 1.0 kPa.
Total wind force. F = pressure × area = 1,000 × (30 × 200) = 6.0 × 10⁶ N.
Where it acts. For uniform pressure, the resultant acts at mid-height, 100 m up.
Overturning moment about the base. M = F × 100 = 6.0 × 10⁸ N m.
Columns on the outer faces, 30 m apart. The columns resist with a couple: tension on the windward side, compression on the leeward side. Each side must carry M/d = 6.0 × 10⁸ / 30 = 2.0 × 10⁷ N.
Columns only in a central core, 10 m apart. Now each side carries 6.0 × 10⁸ / 10 = 6.0 × 10⁷ N, three times as much.
Stiffness improves even more. For the same total column area split between two faces, resistance to bending grows with the square of the distance between them, so the 30 m layout is (30/10)² = 9 times stiffer than the 10 m one. Moving material outward is the core idea of the tube.
What mistakes do students make with this kind of problem?
Putting the wind force at the top. A uniform pressure has its resultant at mid-height.
Forgetting the perpendicular distance. Torque is force times the perpendicular distance from the pivot.
Thinking the model is the whole story. Real tubes do not act perfectly as one piece, which is why Khan added diagonals and bundles.
The JEE (Main) 2026 syllabus covers moment of a force, torque and equilibrium of rigid bodies in Unit 5. Building design itself is beyond the syllabus. For practice, try our guide to torque and equilibrium with ladders.
If moments still feel abstract, the Fermi Concept Library has every concept in physics with its own lesson, including torque and equilibrium.
What can a student take from Khan's way of working?
Find the controlling load. He asked which force really sets the design, and the answer was wind.
Treat economy as part of elegance. His systems used less steel for the same job.
Teach to think. Princeton notes that many of his ideas grew from working with students.
Next step: redo the example for a 45 m wide building of the same height and pressure, and compare the column forces.
